The
Cambrian
sequence on the type section of the Quebrada de La Flecha (Southern
extreme of the Argentine Eastern Precordillera)
ALDO L. BANCHIG AND MARIANA M. RAVIOLO
According to structural stratigraphic and morphological features, the
geological province of the Argentine Precordillera has been divided into three
subprovinces (Western, Central and Eastern Precordillera).
Eastern Precordillera
The Eastern Precordillera is integrated by Villicum to the north,
Sierra Chica de Zonda (to central part), Pedernal and Loma de los Pozos
ranges (to the south), and few other smaller hills and ranges. They are
developed on some 200 km, with a general N-S alignement. The central sector is
topographically the higher.
This mountain system is located 10 km west of San Juan city and
separates the valleys of Zonda to the west and Tulum to the east. Good roads
let us arrive to the north of the Sierra Chica de Zonda (Ullúm dump) while
others do cut across of the central sector of the range through the Quebrada
de Zonda. Towards the north it is possible to reach the sourthern sector of
the Villicum range by the National Road N° 40. Going to the south
two localities can be visited: Pedernal Range (Road N° 34),10 km
to the south of Los Berros Locality, and a second: Quebrada de la La Flecha,
located 8 km to the north of the Los Cerros (mining road) (Fig 1).
The Eastern Precordillera has been for a long time known by the
excellent limestones Lower Paleozoic outcrops, and they were known as
"San Juan Limestones". Harrington y Leanza, (1943), had demostrated
at first time the partial corresponding of the limestones to the Cambrian.
Borrello (1962); Baldis (1981 a, b); Baldis et al. (1982); Baldis y Bordonaro
(1981,1984,1985); Beresi (1986) and Bordonaro (1980), carried out
investigations on the stratigraphic record of Precordilleran limestones.
Historical
background on the
Cambrian and Ordovician of
Eastern Precordillera
The limestones of the Precordillera are known from Kayser (1867);
Stelzner (1873), but it was included in the Ordovician by Kobayashi (1937);
whorhas so-called "San Juan Limestones". Harrington and Leanza
(1943), based on trilobites asigned a Cambrian age to part of the sequence.
Harrington and Leanza (1957), continued using the name "San Juan
Limestones" for all of the carbonatic secuence.
Borrello (1962), identified fosiliferous rocks of Cambrian age and he
separated them from the Ordovician limestones, naming them as "La Laja
Limestones Formation".
Bordonaro (1980,1986), carried out a detailed stratigraphic study in
Lower and Middle Cambrian rocks of La Laja Formation, calling as "Zonda
Formation" the non fosiliferous dolomites which separated the La Laja
Formation and San Juan Formation.
Later, Baldis et al. (1981b),
subdivided the Zonda Formation, and recognized an interval with stromatolites,
calling it as "La Flecha Formation" for the southern Precordillera
outcrops, while in
Keller et al. (1994), identified a
new stratigraphyc unit in the lowermost part of San Juan Formation, calling it
as "La Silla Formarion", whose age comprises Late Cambrian (upper Trempealeau) to uppermost
Tremadoc (deltifer zone). (Fig. 2)
Thus the discovering of new fossils of Lower and Middle Cambrian age
and the stratigraphical criteria applied to the Upper Cambrian - Lower
Ordovician, the latter replaces the original name of "San Juan
Limestones".
Eastern Precordillera: Tectonic Framework
The Geological Province of Eastern Precordillera (Harrington and
Leanza,1957), includes the following ranges: Huaco, Las Salinas, Niquivil,
Tucunuco; Mógna, Víllicum, Loma de las Tapias, Chica de Zonda, Pedernal and Acequión.
They are presented like assymetrical anticlines with axes of N-S strike
-associated to western, vertical and reversed limbs -. The vergency of the
main structures is west, while an increasing degree of stress can be seen
towards the southern areas. These thrusts are represented by the presence of
many inverse faults plain plunging to the east, while towards south of La
Flecha gulch are more frequent, resulting in the imbrication and recurrente of
the Paleozoic sequences, (Baldis et al. 1990). (Fig. 4-5)
Regional Geology
The Eastern Precordillera is a high-level thrust-and-fold with a thick
Lower Palaeozoic sedimentary succesion that includes many and different
depositional environments. Cambrian to Devonian sediments show a marine
depositional environment, whereas the rest of the Neopalaeozoic succesion is
particulary influenced by sedimentation in a continental environment. Red
sediments of continental environment (Paleogene-Neogene), completes the
stratigraphycal scheme associated to modern sedimentary infilling.
(Fig. 5)
Carbonatic rocks of the Eastern Precordillera are exclusively Cambrian
and Ordovician, reaching up to 2500 m in thickness, (Baldis and Bordonaro,
1981). This secuence is represented by limestones and dolomites of a shallow
wide platform comprising the east and central part of the Precordillera. (Fig.
2)
Until 1979, only La Laja and Villicum Formation were recognized as
Cambrian in age, while San Juan Formation was interpreted as Ordovician.
Bordonaro (1980), proposed the name of Zonda Formation for the dolomites of
Chica de Zonda and Villicum ranges (assigned to the Upper Cambrian only by
stratigraphic criteria). The same author grouped all those materials in the
Marquesado Group: (Fig. 2)
Marquesado
Zonda Fm.
Group
(Boxdonaro,1980)
(Bordonaro,1980)
La Laja Fm.
/ / /
Villicum Fm.
(Borrello,1962)
(Borrello,1969)
The
recognition of the lower boundary of San Juan Formation was always doubtful
due to the type of sedimentary transition. Baldis et al. (1981) carried out
detailed studies in the type locality and recognized two new equivalent
formational units that draw the transition between the Cambrian and
Ordovician limestones, including them in the Matagusanos Group. (Fig.
N°2)
La Flecha Section
The Quebrada of La Flecha cuts transversally the Eastern Precordillera
from East to West, fact that helps on a good observation of the excellent
Section with nearly vertical plunged beds. The Cambro-Ordovician transition
will be observed as we detail below:
Base: La
Laja
Formation: (Fig.
2)
Its name comes from the Quebrada de La Laja (Borrello,1962), locality
where the first Cambrian faunas were described for the wholePrecordillera (Harrington and Leanza,1943). This rocks
crop out on the western flank of the
Sierra Chica de Zonda and Villicum.
The lower boundary is unknown due to tecnonic disturbance. The upper
boundary is drawn at the transition from limestones to the dolomites of the
Zonda Formation (Bordonaro, 1980).
The age comprises Lower to Middle Cambrian (Bordonaro 1980, 1986).
Zonda Formation: (Fig.
2)
Bordonaro (1980) named these
rocks, stablishing the southern flank of the Quebrada de Zonda as the type
section. Zonda Formation is lithollogically represented by dolomicrites,
biolaminated deposits, intraformational conglomerates and mudstones with mud
cracks. The stromatolites are scarce and belong to the LLH-type (Logan et al. 1964). Small silicified
mud mounds also occur.
The lower boundary is given by the transition from limestones of the La
Laja Formation to the dolomites of the Zonda Formation.The upper boundary was
established by the sudden occurrence of abundant stromatolites at the base
of La Flecha Formation.
The age of this formation is
assignated by stratigraphical criterion, since in the top of La Laja Formation
in the Sierra Chica de Zonda were found fossils indicating an uppermost Middle
Cambrian age (Bordonaro,1980, 1986). Trilobites of the lowermost part of La
Flecha Formation indicates an Franconian age; therefore the age of the Zonda
Formation would be comprised between the Middle/upper Cambrian boundary and
the lower part of the Franconian.
The thickness of the Zonda Formation varies between 300 and 400 m both
in the Sierra Chica de Zonda and Villicum range.
La Flecha Formation: (Fig. 2)
No formal boudaries have been established in the base of section since
it is cutted by a thrust, (Baldis, et al. 1981 a,b). ). Keller et al. (1994) proposed the lower boundary where the first beds with abundant
stromatolites or thrombolites occur (LLH- and SH-types of Logan et al. 1964). In some sections
the boundary can be drawn by the change of white to brown dolomites.
The upper boundary was
established in the interval where the contents of stromatolites rapidly
decreases and the limestones start to predominate over the dolomitic rocks.
The age of the La Flecha
Formation was stablished with trilobites from the base of the formation in the
type locality in the Quebrada de La Flecha: Plethopeltis
cf. Saratogensis and according to Ludvigsen y
Westrop (1983) y Ludvigsen et al. (1989), P. Saratogensis indicates a late Franconian age. In the middle of the section was found
Stenopilus
convergens (Raymond), which is characteristic of the
The
thickness of La Flecha Formation in the type section is about 400 m . In
Guandacol area 600 m were reported at the Cerro La Silla 730 m were measured
which is the maximun thickness recorded for this Fomation.
La
Flecha Formation is composed of small-scale shallowing-upward cycles (1m -
5m), which exhibit a great variety of stromatolites, thrombolites and
associated structures (Baldis et al. 1981a,
Armella 1989a,b; Cañas 1990), together with subtidal to supratidal
lithologies, characteristic of arid tidal flats (Keller et al. 1989). Early diagenetic dolomites are most abundant and it
is frecuent the replacement of biogenetic structures as well as entire oolite
beds by chert and chalcedony. Limestones or clacareous dolomites are scarce.
La
Flecha Formation was eary defined as a succession of limestones and dolomites
with abundant stromatolites, thrombolites and cryptalgal laminites
("Stratifera" de Baldis et al. 1981
a). The relative abundance of these biogenic structures was used to subdivide
this formation into three members: (Los Berros, Arrecifal and Cañada Honda).
Keller et al. (1994), proposed the
elimination of the names "San Roque Formation" and "Los
Sapitos formation" and they recommended the use of the name "La
Flecha Formation" (in the sense of Baldis et al. 1981b) throughout the Precordillera.
Top: La Silla
Formation. (Fig.
2)
Keller et al, (1994),
Age:
Upper Cambrian (uppermost Trempealeau) to Upper Tremadoc (deltifer zone).
Algal
biocycles associated to La Flecha Formation (Upper Cambrian)
The
stromatolitic sequence of La Flecha was first described by Baldis (1981 a). He
pointed out a cyclothematic arrangement and the continuous association of
stromatolites and thrombolites, indicating changes in the biogenetical
conditions.
Baldis et al.
(1981b), carried out a study on the sector of dispersion of the
stromatolites and presented the analysis of the cyclothemes. He defined La
Flecha Formation and characterized the depositional environment as hipersaline
in a high stability condition. The process was iniciated previously in Zonda
Formation, whose sedimenta indicate a period of regression changing to a
bottom stability period.
Baldis
et a!. (1985), stablished the
secuenciality an idealized algal biocycle with a basal hemicycle associated to
chemical sedimentation. Also to the top a hemicycle characterized by
biological sedimentation-characterized by limestones-planar
stromatolites-domal stromatolites and thrombolites.
Baldis
et al. (1985),
concluded that the algal biocycles of Precordillera were a result of the
fluctuation on the water depth changing from shallow conditions in the basal
section to deeper conditions in the upper section of biocycle.
Armella (1989a), also stablished the trhombolitical microfacies of an ideal biocycle for the La Flecha Formation, identifying eigth microfacies:
MF-1:
Steril micritic mud.
MF-2:
Planar and domal stromatolites of slow relief, with Stratifera, Irregularia, Collenia, Paniscollenia and Weedia.
MF-3:
Domal and composed stromatolites: with Collenia
and Criptozoon.
MF-4: Mantle trombolites.
MF-5: Spheric trombolites.
MF-6: High
relief trombolites.
MF-7:
Microfacies of interthrombolitic channel.
MF-8:
Microfacies ofinfill interthrombolitic no channalized.
The
same author concluded that the combination of the eight microfacies indicates
a ciclycal subsident feature for the Cambro-Ordovician basin. This showed
biocycles that generated slightly transgresive episodes, where the rate of
subsidence was equal or bigger than the biosedimentation, characterized by an
sharp regression on top of each cycle.
According
to Keller et al. (1989),
The Upper Cambrian sedirrients of La Flecha Formation, consist mainly of
peritidal cyclic carbonates. The depositional environment extended from the
shallow subtidal to the supratidal, sometimes even calcretes were formed.
Typical subtidal units show thrombolites, oolites and lithoclastic limestones.
The intertidal shows a great variety of stromatolitic associations, among
which for the first time mudmounds and fenestral mudmounds of a peritidal origin. Supratidal
sediments, mostly mudstones, ehibit mudcracks, tepee-structures and some diagenetic evaporites. The
sediments and their
arrangement in shallowing-upward cycles represent an acient sabkha system
developed on a stable shelf, probably the western passive continental margin
of Gondwana.
The
shallowing-upward cycles (according
to Keller et al. 1989)
The
carbonates of the La Flecha Formation shows well developed cyclicity, which
was descibed by Baldis et al. (1981
a,1984). Keller et al. 1989,
also considered the accopmanying sediments, where the cyclicity becomes more pronounced.
The
cycles normally start with a transgression over a pre-existing, lithified
topography with only a smooth relief. The basal oolites and lithoclastic
graintones are the remnants of the transgression and are either preserved as
laterally persistent sheets or in erosional and dessication structures such as
scours, mudcracks and tepees. Laterally, these high energy deposits interfinger
with thrombolites, which grew under subtidal conditions. The lower intertidal
unit comprises most of the LLH-type stromatolites. Wave and water activity
determine, whether the accompanying sediment is of low (mud) or high energy
origin (oolites, lithoclasts). They grade upward into more stratified, laminated
deposits (microbial laminites, mudstones) of the upper intertidal zone. Here we
find many sedimentary structures higly indicative of a tidal flat
environment: mudcracks, tepees and fenestral fabrics. Upwards, storm-deposited
mud on supratidal flats is rapidly lithified and often fractured into poligons
by desiccation or growth of evaporite minerals. This is the zone, where flat
pebble breccias form and the majority of the evaporites is precipitated within
the sediment column. Finally, longterm exposure leads to the formation
calcretes, which arrange only rocks formed in a terrestrial environment. (Fig
6)
In
the La Flecha Formation more than 100 superimposed cycles were found. An
autocyclic model was proposed for the origin of the shallowing-upward cycles:
Subtidal and intertidal sedimentation exceeded the subsidence (or sea level
rise) of the eastern Precordillera resulting in a regression. This led to a
supratidal or even terrestrial environment with a low sedimentation rate or
even nondeposition. Continuous subsidence resulted in a relative fast
transgression which was balanced by sedimentation as soon as subtidal to
intertidal organisms (e.g. thrombolites and stromatolites) resettled the subtidal to intertidal environment.
In
the Precordillera, the Grand Cycles comprise the lower and middle Cambrian
strata (Baldis
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